The present invention relates to electrical lapping guides for use in machining workpieces to tight tolerances.
Achieving tight tolerances in the machining of workpieces is a demanding endeavor, but nonetheless necessary in many situations. It is of particular interest in the lapping of thin-film magnetic devices, such as magnetic read/write heads. These heads are typically produced employing thin film deposition techniques. In a typical process, a plurality of heads are formed in a grid on a substrate. The substrate is then cut to produce several bars, with one row of heads in a side-by-side relationship on each bar. The pole tips of the head are oriented and extend out toward a first plane of the bar, called the "air bearing surface" (ABS). The extension length of the pole tips of each head toward the air bearing surface defines the "throat height" of such device. Throat height may be adjusted by lapping the first plane of the bar to a required specification. It is preferable to lap throat height down to very tight tolerances in order to establish adequate performance for high-efficiency recording heads.
It is common practice to use an electrical lapping guide (ELG) during lapping of the air bearing surface of the bar in order to accurately establish throat height. A simple form of ELG is shown in the schematic of FIG. 1a. Here a resistor, such as a uniform resistive layer deposited on the surface of a thin film head wafer, is oriented such that the lower edge of the resistor is parallel with the surface to be lapped (the ABS). Typically, the upper edge of the resistor is located beyond the desired lap plane (DLP) where the lapping process would be desired to terminate, and perhaps extends beyond the zero throat height plane (ZTH).
As lapping proceeds, the resistor will gradually be made narrower with a corresponding increase in its resistance. Where the initial width of the resistor is known, and if its upper edge were accurately located relative to the zero throat height plane, then this simple ELG would give all the information required for controlling the lapping process. Zero throat height is determined by a photoresist layer, but we have found it difficult to pattern the resist layer so that its upper edge is positioned exactly along the zero throat height plane.
Another type of ELG is shown schematically in FIG. 1b, where one common leg is coupled to four break point conductors, and where each conductor provides an electrical lead for continuity testing with the common leg. These four conductors intersect with the common leg at slightly different positions with respect to the zero throat height plane. This establishes a plurality of "break-points" assigned to respective throat heights. Initially, these conductors are electrically connected. As lapping proceeds, the break point closest to the air bearing surface is the first to be broken. Such breaking is detected by monitoring continuity through the common leg and the first conductor. After further lapping, the second break point is broken, which can be detected by monitoring continuity through the common leg and the second conductor, and so on. By checking for electrical continuity between the various conductors and the common leg, lapping depth can be readily determined.
This latter type of ELG can be made very accurate because the position of each conductivity break point can be determined by a carefully controlled photoresist process. The photoresist can be deposited and patterned at the same time as the first insulation layer, thus positioning the break points accurately with respect to the zero throat height plane.
The configuration of FIG. 1b may be rearranged into the structure shown schematically in FIG. 1c. Here, a respective one of four resistors (rc-rf) has been added to each of four break point conductors, where these conductors commonly terminate at one end at a common test point B and are connected at their other ends to the common leg at respectively different positions c-f with respect to the zero throat height plane. The common leg terminates at test point A. In this arrangement, only two leads are required for continuity checking no matter how many break points are used. During lapping, the severing of a break point can be determined simply by detecting a change of resistance between the two test points, A, B. Hence, as lapping of the air bearing surface proceeds, break points c, d, e and f are sequentially broken so as to sequentially and effectively remove resistors rc-rf from the A-B measuring circuit. As a result, as each of the break points is broken, a discrete increase in the measured resistance between leads A, B will be detected. These resistive changes will be indicative of lapping depth into the air bearing surface.
One known configuration of a thin film read/write head includes a first and second thin film head within a single structural unit (called a "slider") which is shown in FIG. 1d. The poles of each head extend into respective "rails" C, D protruding from the bottom of the slider surface. Although not shown, it will be appreciated that a view from the air bearing surface of the slider of FIG. 1d will reveal each thin film head having an upper pole piece P2 and a lower pole piece P1 whose tips are separated by an insulating gap and extend to the air bearing surface. Each head of a two head slider will also be provided with a coil, transducer, or other like device to enable reading/writing during operation of the head. A plurality of sliders will lie side-by-side on a single bar which has been sliced from a wafer during fabrication.